Abstract

5004

The growth of tumors can be inhibited by chemotherapy-induced long-term cell cycle arrest, which is frequently associated with cellular senescence. Many studies have demonstrated that following DNA damage, p53 gene is the key events in regulation of cells to undergo senescence or apoptosis. However, we previously showed that in human colon cancer cell lines, inactivation of MGMT by BG, an inhibitor of MGMT, triggered BCNU-induced DNA damaged cells in a permanent G2/M arrest due to an inhibition of mitotic entry. This cellular event was independent of p53 status. To determine the contribution of MGMT to the regulation of cell proliferation in response to TMZ-induced DNA damage, we used glioma cell lines T98G and LN 18 (MGMT proficient and p53 mutant) as well as U87 and D54 (MGMT deficient p53 wt). Both U87 and D54 cell lines responded to damage by undergoing a G2/M arrest beginning 48 hr after TMZ at IC50 concentrations with or without BG co-treatment. The cells displayed p53 and p21 associated senescence over a 5-day period, however, very few of these cells underwent apoptosis. In contrast, the majority of T98G cells expressing high levels of MGMT) arrested in G2/M that was observed only in cells treated with BG and TMZ. Flattened giant cells appeared and over 60% of cells showed SA-β-gal expression 48 hrs after treatment. At day 5, over 50% of cells underwent apoptosis. In these cells, CHK1 was downregulated and cdc2/Cyclin B1 proteins were inactivated, whereas the CDK2 and Cyclin E remained consistent in cells with and without drug treatments. These data revealed that these cells were in the G2/M phase of the cell cycle and possibly proceeded through an unscheduled premature mitosis, which develops into a mitotic catastrophe, and to apoptosis. Thus, MGMT appears to be one of components involved in triggering cells to undergo senescence, along with mitotic catastrophe and apoptosis, a major antiproliferative effect of chemotherapy. The role of MGMT in the signal transduction program leading to irreversible cell cycle arrest after DNA damage need further invastigation.